An X-ray CT apparatus is designed to rotate an X-ray source and an X-ray detector around the body axis of an object, cause the X-ray source to irradiate the object with X-rays, and reconstruct a tomographic image based on the projection data obtained by detecting the X-rays transmitted through the object using the X-ray detector. This apparatus plays an important role in various medical practices such as disease diagnoses and medical and surgical planning.
The projection data obtained by an X-ray CT apparatus is discrete data, and hence aliasing artifacts occur more or less in reconstructed images. As methods of preventing the occurrence of such artifacts, there have been proposed a method (flying focal spot technique) of apparently reducing a channel pitch to ½ by executing data acquisition concerning the same slice plane for each focus by using an X-ray source having two focuses, an offset method (QQ offset technique) of offsetting the center position of the X-ray detector relative to the imaging center line connecting the focus and the rotation axis by a distance corresponding to a fraction of a channel pitch, and a method of applying an LPF (Low Pass Filter) to projection data.
The above flying focal spot technique requires a dedicated X-ray tube, and hence it is not easy to introduce this technique. In addition, according to the QQ offset technique, when using an apparatus having a cone angle such as an AD (Area Detecting) CT, since real data and counter data do not exist on the same plane around this cone angle, it is not possible to obtain ideal counter data. Under the circumstances, a method using a low pass filter is often used as a main approach to reduce aliasing artifacts.
Although artifacts are reduced by uniformly applying a low pass filter to the entire region of projection data, the resolution of a reconstructed image decreases to impair granularity.
It is an object to provide an X-ray CT apparatus and image processing apparatus which can reduce artifacts in a reconstructed image while maintaining the resolution of the reconstructed image by using a simple method.